Nuclear power reactors are well known and are discussed, for example, by M. M. El-Wakil in "Nuclear Power Engineering" McGraw-Hill Book Company Inc., 1962.
In a known type of nuclear power reactor, for example, as used in the Dresden Nuclear Power Station near Chicago, Ill., the reactor core is of the heterogenous type. In such reactors the nuclear fuel comprises elongated rods formed of sealed cladding tubes of suitable material, such as a zirconium alloy, containing uranium oxide and/or plutonium oxide as the nuclear fuel, for example, as shown in U.S. Pat. No. 3,365,371. A number of such fuel rods are grouped together and contained in an open-ended tubular flow channel to form a separately removable fuel assembly or bundle as shown, for example, in U.S. Pat. No. 3,431,170. A sufficient number of fuel assemblies are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
A typical fuel assembly is formed by an array of spaced fuel rods supported between upper and lower tie plates, the rods being several feet in length, on the order of one-half inch in diameter and spaced from one another by a fraction of an inch. To provide proper coolant flow past the fuel rods it is important to maintain the rods in spaced position and restrain them from bowing and vibrating during reactor operation. A plurality of fuel rod spacers spaced along the length of the fuel assembly are provided for this purpose.
Design considerations of such fuel rod spacers include the following: retention of rod-to-rod spacing; retention of fuel assembly shape; allowance for fuel rod thermal expansion; restriction of fuel rod vibration; ease of fuel bundle assembly; minimization of contact areas between the spacer and fuel rods; maintenance of structural integrity of the spacer under normal and abnormal (such as seismic) loads; minimization of reactor coolant flow distortion and restriction; maximization of thermal limits; minimization of parasitic neutron absorption; minimization of manufacturing costs including adaptation to automated production. Thus the need to provide such fuel rod spacers creates several significant problems two of which are parasitic neutron absorption and coolant flow restriction or pressure drop.
Any material, in addition to the nuclear fuel, that must be used in the construction of the reactor core unproductively absorbs neutrons and thus reduces reactivity with the result that an additional compensating amount of fuel must be provided. The amount of such parasitic neutron absorption is a function of the amount of the non-fuel material, of its neutron absorption characteristics, that is, its neutron absorption cross section, and of the neutron flux density to which it is exposed.
To remove the heat from the nuclear fuel, pressurized coolant is forced through the fuel assemblies of the reactor core. The fuel rod spacers in the assemblies act as coolant flow restrictors and cause an undesirable though inevitable coolant flow pressure drop. To maintain proper cooling of the fuel rods along their length and to minimize the required coolant pumping power it is desirable that spacer coolant flow resistance be minimized. The flow resistance of a spacer is a strong function of its projected or "shadow" area. Therefore, the flow resistance of a spacer can be minimized by minimizing the projected area of the structure of the spacer.
As a practical matter the desire to minimize both parasitic neutron absorption and coolant flow restriction presents a conflict in fuel rod spacer design.
To minimize coolant flow restriction, spacer members should be thin and of minimal cross section area. However, very thin members must be formed of high strength material to provide suitable spacer strength. Also, high strength material with suitable resilience characteristics must be used for any spring member portions of the spacer. It is found that suitable such materials have relatively high neutron absorption characteristics.
On the other hand, materials of desirably low neutron absorption characteristics are found to be of relatively low strength, difficult to form and lacking the resiliency desired for the spring member portions of the spacer.
An approach toward the resolution of the foregoing design conflict is a "composite" spacer wherein the structural members are formed of a material having a low neutron absorption cross section and the spring members thereof are separately formed of suitably resilient material whereby the amount of high neutron absorption cross section material is minimized.
A variety of such fuel rod spacers have been proposed and used. An example is shown in U.S. Pat. No. 3,654,077. The spacer shown therein (especially the embodiment of FIGS. 5 and 6 thereof) has enjoyed long commercial success. In the spacer thereof the peripheral support member and the divider members are formed of low neutron cross section material such as zirconium. The divider members are skeletonized, i.e., formed with cutouts, to further reduce neutron loss. To minimize the amount of high neutron cross section spring material in the spacer, a single spring member projects into each of the fuel rod passages, the springs being in the form of four-sided assemblies.
The core of a large nuclear power reactor typically contains in the order of 800 fuel assemblies each of which may have seven spacers. Thus it can be appreciated that even small decreases in spacer flow resistance and neutron capture can have a significant effect on the core as a whole.
Decreased flow resistance means that less coolant recirculation pumping power is needed. For example, in a 1000 MWe (megawatt electric) plant, a reduction of core flow resistance of one psi can save as much as 350 kWe of power.
Decreased parasitic neutron absorption in the spacers means that less core reactivity is needed for a given power output. In other words, fuel of lower enrichment can be used. For example, a 0.01 percent decrease in enrichment can lower the cost of the fuel by in the order of $1000.00 per assembly.
An object of this invention is a nuclear fuel element spacer providing decreased coolant flow resistance and decreased neutron absorption.
Another object is to decrease the projected area of a spacer structure.
Another object is to minimize changes in the cross section area of the spacer structure.
Another object is to securely retain the spring members of a spacer.
Another object is to avoid projection of springs or other spacer structure above and below the planes formed by the upper and lower edges of the cross-laced divider members of the spacer.